It looks like the impression left from a hole saw with the drill bit hole in the center.
That's an interesting observation and certainly within the realm of possability. The hole edge is very neat and the (presumed original) firewall paint extends into the hole. It does not appear to be raw cut like with a hole saw but more like a factory die cut. There's no impression into the insulation pad at the circumference of the hole like a hole saw would make. The car was certainly hot-rodded in its past and it might have been cut for an oil pressure line or some such added gauge.
I saw one map of early Mustang firewall in a
https://www.vintage-mustang.com/threads/firewall-holes.949033/#lg=thread-949033&slide=0 VMF forum post that was apparently lifted from a Mustang Monthly article.
I cannot vouch for the information but it indicated an oil pressure line hole. Possibly a Shelby prototype thing?
But the important information from the perspective of Concours Mustang is whether the hole was factory-made. If the conclusion is no, it is not factory-made, then I will continue to investigate other sites of Mustang lore.
I greatly value your expertise! - Jim